Practical_Antenna_Handbook_0071639586

C h a p t e r 1 8 : a n t e n n a s f o r 1 6 0 M e t e r s 417
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XMTR
Figure 18.6 The inverted-L.
Because the horizontal portion of the wire extends in a single direction from the
vertical portion, it causes a very slight distortion in the azimuthal radiation pattern of
the antenna.
Far more important, however, is this: Because it is a member of the family of vertical
monopoles, the inverted-L will not work well without an adequate ground system
under it. Chapter 30 and the beginning of this chapter cover RF grounding in more detail,
but here’s a summary of what’s important for an inverted-L:
• Run 12 to 50 radials of 50 to 150 ft, depending on the available space. A good
rule of thumb is to put down as many equally spaced radials as necessary to
have their far tips be no more than about 0.05l apart. On 160, that’s about 8-ft
spacing between tips. As explained in Chap. 9, the longer you can make your
radials, the more you should use, and the higher your overall antenna and
ground system efficiency will be.
• The center of the radial circle should be at the feedpoint of the inverted-L. In
other words, tie the inner end of all the radials to the shield side of the coaxial
transmission line, using a chassis-mount SO-239 UHF receptacle or whatever
best matches the connector on your feedline. Tape the joint with electrical tape
and spray it with clear acrylic spray. Cover the entire joint with a small plastic